This invention relates generally to inverter-fed machine drive systems, and more specifically, to an improved microcomputer-based control apparatus for controlling inverter switching device conduction in accordance with operator commands to achieve improved regulation of machine torque and frequency.
The ability to control the torque and frequency of a polyphase alternating current machine of either the synchronous or induction type is very useful. Often, it is desirable to adjust machine frequency and torque in response to rapidly changing load conditions. Since the torque and frequency of a polyphase alternating current machine are dependent on the amplitude and frequency of machine terminal voltage, conditioning, that is, varying the amplitude and frequency of machine terminal voltage in accordance with external commands allows machine torque and frequency to be regulated accordingly.
Conditioning of the alternating current supplied to a polyphase alternating current machine is commonly accomplished by use of a power converter, such as an inverter coupled between a low frequency, (e.g. 0-60 Hz.) potential source and the polyphase machine. Typically, the inverter comprises a plurality of pairs of switching devices, either high current transistors or thyristors, with the switching devices of each pair coupled in series aiding fashion, and each of the pairs coupled across the low frequency potential source and coupled at the junction between serially coupled switching devices, respectively, to an associated machine phase, respectively. When the switching devices of each of the pairs are rendered conductive in a prescribed sequence, alternating current is supplied to the machine causing it to become excited and commence rotation. By controlling the conduction duration of each of the inverter switching devices, the amplitude of power converter output voltage can be regulated accordingly. Control of the frequency at which the power inverter switching devices are rendered conductive allows control of machine frequency.
Heretofore, various techniques have been described for controlling inverter switching device conduction duration and frequency. The most common is that of pulse width modulation, often abbreviated as PWM, whereby the switching devices of each inverter pair are alternately rendered conductive, and the pairs of switching devices in the inverter are switched, in sequence, at a frequency greater than twice the desired machine frequency. Rendering the inverter switching devices conductive in this manner results in each phase component of inverter output voltage exhibiting a waveform comprised of a series of alternately positive and negative pulses. The width of pulses is proportional to the output voltage magnitude and the fundamental frequency of the pulse waveform is proportional to inverter output frequency.
In the past, pulse width modulation control of inverter switching device conduction has been accomplished by use of dedicated analog or digital hardware. Unfortunately, such dedicated hardware is usable only in conjunction with a selected class of alternating current machine drive systems for which the hardware was designed. To adapt such dedicated hardware to different classes of machine drive systems is often time-consuming, if possible at all.
In contrast, the present invention concerns an improved, microcomputer-based, control apparatus for a machine drive system which achieves pulse width modulation inverter operation with considerable simplification in hardware. The present invention may also be adapted for use on different classes of machine drive systems with only a modification of microcomputer software.
It is an object of the present invention to provide a microcomputer-based control apparatus for an inverter-fed machine drive system which regulates machine frequency and torque;
It is another object of the present invention to provide a microcomputer-based control apparatus for an inverter-fed machine drive system which regulates the duration and sequence of inverter switching device conduction in accordance with inverter pulse width modulation signal pulses and half cycle polarity signal pulses, the inverter pulse width modulation signal pulses data produced by equilateral trangulation during intervals of low machine frequency and by synthesis from discrete stored wave patterns selected in accordance with either the selected harmonic elimination method or the machine ripple current minimization method during intervals of other than low machine frequency, thereby assuring smooth regulation of machine torque and frequency.